Add what to what? The original circuit consisted of a 10 volt source and a 10 ohm resistor (or something else that limited the current to 1 amp.
If you add 10 more ohms in series with the original 10 ohms, the current will fall to 0.5 amp.
And that 5 watts will be divided equally between each of the two 10 ohm resistors, 2.5 watts each.
As long as tow 10 ohm resistors are in series, each needs to be capable of getting rid of at least 2.5 watts.